Catalyst for exhaust gas purification, process for producing the same, and method of purifying exhaust gas

FIG. 1 illustrates a schematic cross-sectional view of an exhaust gas purifying catalyst of this embodiment. This exhaust gas purifying catalyst is constituted by a honeycomb-shaped monolithic substrate 1 , and a coating layer 2 formed on the surface of the monolithic substrate 1 . The coating layer 2 is constituted mainly by an Al 2 O 3 powder 20 and a TiO 2 powder 21 , and an Rh/ZrO 2 powder 22 , which is made by loading Rh on the ZrO 2 powder, a CeO 2 —ZrO 2 composite oxide powder 23 , Pt 24 , Ba 25 , K 26 and Li 27 are loaded.

Hereinafter, a production process of this catalyst will be described instead of the detailed description of the arrangements.

A rhodium nitrate aqueous solution having a predetermined concentration was impregnated into a ZrO 2 powder in a predetermined amount, was dried and burned so that an Rh/ZrO 2 powder was prepared. On the Rh/ZrO 2 powder, 0.42% by weight of Rh is loaded.

The resulting Rh/ZrO 2 powder was mixed with a γ-Al 2 O 3 powder, a rutile type TiO 2 powder and a CeO 2 —ZrO 2 composite oxide powder so that a support powder was prepared. The mixing ratio was, by weight ratio, Al 2 O 3 :TiO 2 :Rh/ZrO 2 :CeO 2 —ZrO 2 =1:1:0.1:0.2.

After this support powder was mixed well, it was mixed with water and a trace amount of an alumina sol so that a slurry was prepared, was coated on a surface of a cordierite monolithic support having a volume of 1.3 liter, and was dried at 250° C. for 15 minutes so that a coating layer was formed. The coating layer was formed in an amount of 270 g with respect to 1 liter of the monolithic substrate.

A barium acetate aqueous solution having a predetermined concentration was impregnated into the monolithic catalyst having the coating layer, was dried at 250° C. for 15 minutes, and was thereafter burned at 500° C. for 30 minutes so that Ba was loaded. The loading amount of Ba was 0.2 mole with respect to 1 liter of the monolithic substrate. This was immersed into an aqueous ammonium bicarbonate having a concentration of 15 g/L for 15 minutes, and was dried at 250° C. for 15 minutes so that barium carbonate was made.

Then, a dinitrodiamine platinum nitrate aqueous solution having a predetermined concentration was prepared, the aforementioned monolithic support with Ba loaded was immersed thereinto, was taken up therefrom, was blown off excessive liquid droplets, was dried at 300° C. for 15 minutes, and was burned at 500° C. for 30 minutes so that Pt was loaded. The loading amount of Pt was 2 g with respect to 1 liter of the monolithic substrate.

Moreover, an aqueous solution containing potassium nitrate and lithium nitrate in predetermined concentrations was impregnated into the coating layer in a predetermined amount, was dried at 250° C. for 15 minutes, and was thereafter burned at 500° C. for 30 minutes so that K and Li were loaded, and a catalyst of Example No. 1 was prepared. The loading amounts of K and Li were 0.1 mole, respectively, with respect to 1 liter of monolithic substrate.

The resulting catalyst was installed to an exhaust system of a 1.8 L lean burn engine, and a facilitated durability test, in which an automobile was driven in a pattern simulating an urban driving, by using a gasoline containing 200 ppm of sulfur components for 50 hours. Thereafter, while supplying a lean air-fuel mixture whose A/F=18 or more, an air-fuel mixture of from a stoichiometric point to a rich atmosphere whose A/F=14.6 or less was supplied every dozens of seconds in a pulsating manner, the automobile was run in the 10-15 mode, and the NO x conversions in the driving were measured. The results are illustrated in FIG. 2 .

Except that the mixing ratio was varied to Al 2 O 3 :TiO 2 :Rh/ZrO 2 :CeO 2 —ZrO 2 =1:1:0.2:0.2 by weight ratio, a support was prepared in the same manner as Example No. 1, the respective components were loaded in the same manner so that a catalyst of Example No. 2 was prepared. And, in the same manner as Example No. 1, NO x conversions after the durability test were measured. The results are illustrated in FIG. 2 .

Except that the mixing ratio was varied to Al 2 O 3 :TiO 2 :Rh/ZrO 2 :CeO 2 —ZrO 2 =1:1:0.5:0.2 by weight ratio, a support was prepared in the same manner as Example No. 1, the respective components were loaded in the same manner so that a catalyst of Example No. 3 was prepared. And, in the same manner as Example No. 1, NO x conversions after the durability test were measured. The results are illustrated in FIG. 2 .

Except that the mixing ratio was varied to Al 2 O 3 :TiO 2 :Rh/ZrO 2 :CeO 2 —ZrO 2 =1:1:1:0.2 by weight ratio, a support was prepared in the same manner as Example No. 1, the respective components were loaded in the same manner so that a catalyst of Example No. 4 was prepared. And, in the same manner as Example No. 1, NO x conversions after the durability test were measured. The results are illustrated in FIG. 2 .

Except that the mixing ratio was varied to Al 2 O 3 :TiO 2 :Rh/ZrO 2 :CeO 2 —ZrO 2 =1:1:2:0.2 by weight ratio, a support was prepared in the same manner as Example No. 1, the respective components were loaded in the same manner so that a catalyst of Example No. 5 was prepared. And, in the same manner as Example No. 1, NO x conversions after the durability test were measured. The results are illustrated in FIG. 2 .

Except that the mixing ratio was varied to Al 2 O 3 :TiO 2 :Rh/ZrO 2 :CeO 2 —ZrO 2 =1:1:4:0.2 by weight ratio, a support was prepared in the same manner as Example No. 1, the respective components were loaded in the same manner so that a catalyst of Example No. 6 was prepared. And, in the same manner as Example No. 1, NO x conversions after the durability test were measured. The results are illustrated in FIG. 2 .

Except that an Rh/ZrO 2 powder whose Rh concentration was 0.21% by weight was used, a support was prepared in the same manner as Example No. 1, the respective components were loaded in the same manner so that a catalyst of Example No. 7 was prepared. And, in the same manner as Example No. 1, NO x conversions after the durability test were measured. The results are illustrated in FIG. 2 .

Except that an Rh/ZrO 2 powder whose Rh concentration was 0.08% by weight was, a support was prepared in the same manner as Example No. 1, the respective components were loaded in the same manner so that a catalyst of Example No. 8 was prepared. And, in the same manner as Example No. 1, NO x conversions after the durability test were measured. The results are illustrated in FIG. 2 .

Except that an Rh/ZrO 2 powder whose Rh concentration was 1.25% by weight was used, a support was prepared in the same manner as Example No. 1, the respective components were loaded in the same manner so that a catalyst of Example No. 9 was prepared. And, in the same manner as Example No. 1, NO x conversions after the durability test were measured. The results are illustrated in FIG. 2 .

Except that the Al 2 O 3 powder and the TiO 2 powder were mixed in 10:1 by weight ratio, and that the mixing ratio was varied to the mixed powder: Rh/ZrO 2 :CeO 2 —ZrO 2 =2:0.5:0.2 by weight ratio, a support was prepared in the same manner as Example No. 1, the respective components were loaded in the same manner so that a catalyst of Example No. 10 was prepared. And, in the same manner as Example No. 1, NO x conversions after the durability test were measured. The results are illustrated in FIG. 2 .

Except that the Al 2 O 3 powder and the TiO2 powder were mixed in 4:1 by weight ratio, and that the mixing ratio was varied to the mixed powder: Rh/ZrO 2 :CeO 2 —ZrO 2 =2:0.5:0.2 by weight ratio, a support was prepared in the same manner as Example No. 1, the respective components were loaded in the same manner so that a catalyst of Example No. 11 was prepared. And, in the same manner as Example No. 1, NO x conversions after the durability test were measured. The results are illustrated in FIG. 2 .

Except that the Al 2 O 3 powder and the TiO 2 powder were mixed in 1:4 by weight ratio, and that the mixing ratio was varied to the mixed powder: Rh/ZrO 2 :CeO 2 —ZrO 2 =2:0.5:0.2 by weight ratio, a support was prepared in the same manner as Example No. 1, the respective components were loaded in the same manner so that a catalyst of Example No. 12 was prepared. And, in the same manner as Example No. 1, NO x conversions after the durability test were measured. The results are illustrated in FIG. 2 .

Except that a powder (Rh/ZrO 2 —Ca), in which ZrO 2 included Ca in an amount of 4 mole % and Rh was loaded on ZrO 2 in an amount of 0.42% by weight, was used instead of the Rh/ZrO 2 powder, a support was prepared in the same manner as Example No. 3, and the respective components were loaded in the same manner so that a catalyst of Example No. 13 was prepared. And, in the same manner as Example No. 1, NO x conversions after the durability test were measured. The results are illustrated in FIG. 2 .

Comparative Example No. 1

Except that the Rh/ZrO 2 powder was not used, a support was prepared in the same manner as Example No. 1, and the respective components were loaded in the same manner so that a catalyst of Comparative Example No. 1 was prepared. And, in the same manner as Example No. 1, NO x conversions after the durability test were measured. The results are illustrated in FIG. 2 .

Comparative Example No. 2

Except that the Rh/ZrO 2 powder was not used, and that the Al 2 O 3 powder and the TiO 2 powder were mixed in a ratio of 10:1 by weight, a support was prepared in the same manner as Example No. 1, and the respective components were loaded in the same manner so that a catalyst of Comparative Example No. 2 was prepared. And, in the same manner as Example No. 1, NO x conversions after the durability test were measured. The results are illustrated in FIG. 2 .

Comparative Example No. 3

Except that the Rh/ZrO 2 powder was not used, and that the Al 2 O 3 powder and the TiO 2 powder were mixed in a ratio of 4:1 by weight, a support was prepared in the same manner as Example No. 1, and the respective components were loaded in the same manner so that a catalyst of Comparative Example No. 3 was prepared. And, in the same manner as Example No. 1, NO x conversions after the durability test were measured. The results are illustrated in FIG. 2 .

Comparative Example No. 4

Except that the Rh/ZrO 2 powder was not used, and that the Al 2 O 3 powder and the TiO 2 powder were mixed in a ratio of 1:4 by weight, a support was prepared in the same manner as Example No. 1, and the respective components were loaded in the same manner so that a catalyst of Comparative Example No. 4 was prepared. And, in the same manner as Example No. 1, NO x conversions after the durability test were measured. The results are illustrated in FIG. 2 .

Comparative Example No. 5

Except that the Rh/ZrO 2 powder was not used, a support was prepared in the same manner as Example No. 1, the respective components were thereafter loaded in the same manner, and further a rhodium nitrate aqueous solution having a predetermined concentration was used, and Rh was loaded in an amount equal to the amount contained in the Rh/ZrO 2 powder on the support of Example No. 1 so that a catalyst of Comparative Example No. 5 was prepared. And, in the same manner as Example No. 1, NO x conversions after the durability test were measured. The results are illustrated in FIG. 2 . Namely, this catalyst was free from ZrO 2 .

Comparative Example No. 6

Except that a ZrO 2 powder was used instead of the Rh/ZrO 2 powder, a support was prepared in the same manner as Example No. 1, and the respective components were loaded in the same manner so that a catalyst of Comparative Example No. 6 was prepared. And, in the same manner as Example No. 1, NO x conversions after the durability test were measured. The results are illustrated in FIG. 2 . Namely, this catalyst was free from Rh.

Comparative Example No. 7

Except that a ZrO 2 powder was used instead of the Rh/ZrO 2 powder, a support was prepared in the same manner as Example No. 1, the respective components were thereafter loaded in the same manner, and further a rhodium nitrate aqueous solution having a predetermined concentration was used, and Rh was loaded in an amount equal to the amount contained in the Rh/ZrO 2 powder on the support of Example No. 1 so that a catalyst of Comparative Example No. 7 was prepared. And, in the same manner as Example No. 1, NO x conversions after the durability test were measured. The results are illustrated in FIG. 2 . Namely, in this catalyst, Rh was not necessarily loaded on ZrO 2 .

Evaluation

In Table 1, there is summarized and shown the support arrangements of the catalysts of the aforementioned examples and comparative examples.

According to FIG. 2, it is understood that the catalysts of the respective examples were superior to the catalysts of comparative examples in terms of the NO x purifying capability after the durability test, and it is apparent that it was an effect resulting from mixing the Rh/ZrO 2 powder with the support composed of Al 2 O 3 and TiO 2 . Namely, the present exhaust gas purifying catalyst is extremely good in terms of the durability, and, in accordance with the present exhaust gas purifying catalyst, it is possible to stably reduce and purify the NO x in the exhaust gas for a long period of time.

And, by comparing Example Nos. 1-6, it is understood that there is an optimum range in terms of the Rh/ZrO 2 powder content, and it is understood that it can preferably fall in a range of ¼-½ with respect to the summed amount of Al 2 O 3 and TiO 2 . Further, by comparing Example No. 1 and Example Nos. 7-9, it is understood that the Rh concentration in the Rh/ZrO 2 powder can preferably be around 0.21% by weight. Furthermore, by comparing Example No. 3 and Example Nos. 10-12, it is understood that the mixing ratio of Al 2 O 3 and TiO 2 can preferably be adjacent to 1:1 by weight ratio.

Whilst, by comparing Comparative Example Nos. 5-7, when Rh coexisted with ZrO 2 , it is understood that the NO x purifying capability was improved most. And, by comparing Example No. 1 with Comparative Example No. 7, it is apparent that an exceptionally high NO x purifying conversion was exhibited when Rh was loaded on ZrO 2 .

Moreover, by comparing Example No. 2 and Example No. 13, it is also understood that it was more preferable to use ZrO 2 , which was stabilized by Ca and which was loaded with Rh, than to use simple the Rh/ZrO 2 .

A predetermined amount of a rhodium nitrate aqueous solution having an Rh concentration of 3% by weight was charged into a beaker, which held 100 parts by weight of distilled water therein, and 20 parts by weight of a ZrO 2 powder having a specific surface area of 90 m 2 /g was added thereto while stirring, and was mixed for 1 hour. This mixture solution was filtered by vacuum suctioning, was dried at 110° C., and was thereafter calcined at 250° C. for 1 hour to load Rh in an amount of 0.5% by weight to prepare a Rh/ZrO 2 powder. Hereinafter, this powder is referred to as a powder “A”.

Whilst, an Al chelate (ethyl acetoacetate aluminum di-isopropylate) and a Ti alkoxide (titanium tri-isopropoxide) were dissolved into 2-propanol so that they were Al 2 O 3 :TiO 2 =2:1 by molar ratio, were held at 60° C. while stirring, and were hydrolyzed by adding a predetermined amount of water. After the hydrolysis, they were insulated thermally at 60° C., the resulting gel was aged, and it was thereafter dried under decompression while subjecting it to a vibration. After calcining the dried powder at 500° C., it was burned at 800° C. so that an amorphous Al—Ti composite oxide powder was prepared.

Next, by using a dinitrodiammine platinum hydroxide aqueous solution, 2 g of Pt was loaded on and with respect to 160 g of this Al—Ti composite oxide powder. Further, by using a mixture aqueous solution of barium acetate and potassium acetate, 0.2 mole of Ba and 0.1 mole of K were loaded on and with respect to 160 g of the Al—Ti composite oxide powder. Hereinafter, this powder is referred to as a powder “B”.

50 parts by weight of the powder “A” and 150 parts by weight of the powder “B” were weighed, were charged into a ball mill, and were mixed for 48 hours. This mixture powder was formed by hydrostatic pressure at a forming pressure of 49 MPa, this formed substance was pulverized in a mortar, and was thereafter made to the same particle diameter by a sieve so that pelletized catalysts of from 0.5-1.7 mm approximately in size were prepared. This pelletized catalyst was disposed in an evaluation apparatus, and a model gas of lean 1 set forth in Table 2 was flowed at a space velocity of 100,000 hr −1 at a catalyst-inlet gas temperature of 350° C. so that an initial NO x conversion was measured. The NO x conversion was calculated from an NO x concentration at the inlet of the catalyst and an NO x concentration at the outlet of the catalyst.

Further, this pelletized catalyst was disposed in an evaluation apparatus, and a durability test was carried out by alternately flowing a model gas of lean 2 set forth in Table 2 and a rich model gas while switching the gases (a model gas of lean 2 was flowed for 1 minute, and a rich model gas was flowed for 5 seconds) at a catalyst inlet temperature of 600° C. for 5 hours. Then, the model gas of lean 1 set forth in Table 1 was flowed at a space velocity of 100,000 hr −1 at a catalyst inlet gas temperature of 350° C. so that an NO x conversion after durability was measured.

Furthermore, the temperature of the pelletized catalysts after the durability test was increased from room temperature by a temperature increment rate of 10° C./minute in the aforementioned rich model gas, and the SO 2 elimination behavior was examined by using a mass spectrometer. And, the SO 2 elimination temperature, at which the elimination behavior curve exhibits a peak value, for example, as illustrated in FIG. 3, was measured. The SO 2 elimination temperature was thus measured. These results are set forth in Table 3.

Except that the Rh loading amount of the powder “A” was varied to 0.1% by weight, a pelletized catalyst was prepared in the same manner as Example No. 14, and was similarly measured for the initial and after durability NO x conversions and the SO 2 elimination temperature. These results are set forth in Table 3.

Except that the Rh loading amount of the powder “A” was varied to 2% by weight, a pelletized catalyst was prepared in the same manner as Example No. 14, and was similarly measured for the initial and after durability NO x conversions and the SO 2 elimination temperature. These results are set forth in Table 3.

Except that the Rh loading amount of the powder “A” was varied to 4% by weight, a pelletized catalyst was prepared in the same manner as Example No. 14, and was similarly measured for the initial and after durability NO x conversions and the SO 2 elimination temperature. These results are set forth in Table 3.

Except that the Ba loading amount of the powder “B” was varied to 0.1 mole, a pelletized catalyst was prepared in the same manner as Example No. 14, and was similarly measured for the initial and after durability NO x conversions and the SO 2 elimination temperature. These results are set forth in Table 3.

Except that Mg was loaded instead of Ba in an amount of 0.2 mole in the powder “B”, a pelletized catalyst was prepared in the same manner as Example No. 14, and was similarly measured for the initial and after durability NO x conversions and the SO 2 elimination temperature. These results are set forth in Table 3.

Except that Li was loaded instead of Ba in an amount of 0.1 mole in the powder “B” and K was loaded in an amount of 0.2 mole therein, a pelletized catalyst was prepared in the same manner as Example No. 14, and was similarly measured for the initial and after durability NO x conversions and the SO 2 elimination temperature. These results are set forth in Table 3.

Except that the composition ratio of the Al 2 O 3 powder and the TiO 2 powder was varied to a ratio of 6:1 by molar ratio in the powder “B”, a pelletized catalyst was prepared in the same manner as Example No. 14, and was similarly measured for the initial and after durability NO x conversions and the SO 2 elimination temperature. These results are set forth in Table 3.

Except that the mixing ratio of the powder “A” and the powder “B” was varied to the powder “A”/the powder “B”=90/110 by weight ratio, a pelletized catalyst was prepared in the same manner as Example No. 14, and was similarly measured for the initial and after durability NO x conversions and the SO 2 elimination temperature. These results are set forth in Table 3.

An amorphous Al—Ti composite oxide was prepared in the same manner as Example No. 14. Next, by using a dinitrodiammine platinum hydroxide aqueous solution, 2 g of Pt was loaded on and with respect to 160 g of this Al—Ti composite oxide powder.

150 parts by weight of the resulting powder, 50 parts by weight of the powder “A”, 6 parts by weight of aluminum hydroxide serving as a binder, 30 parts by weight of aluminum nitrate serving as a pH conditioner, and 200 parts by weight of water were mixed so that a slurry was prepared.

Next, a honeycomb-shaped monolithic substrate was prepared, the aforementioned slurry was coated thereon, was dried, and was burned to prepare a coating layer thereon. The coating amount of the coating layer was 240 g with respect to 1 liter of the monolithic substrate. Next, by using a barium acetate aqueous solution, Ba was loaded in the coating layer in an amount of 0.2 mole with respect to 1 liter of the monolithic substrate. And, it was immersed into an ammonium bicarbonate so that Ba was turned into BaCO 3 . Further, by using a dinitrodiammine platinum hydroxide aqueous solution, 1 g of Pt was loaded in the surface portion of the coating layer with respect to 1 liter of the monolithic substrate, finally, by using a potassium acetate aqueous solution, K was loaded in the coating layer in an amount of 0.1 mol with respect to 1 liter of the substrate.

The resulting monolithic catalyst was processed to a diameter of 3 cm and a length of 5 cm in order to make a test piece. Then, this test piece was disposed in an evaluation apparatus, and, in the same manner as Example No. 1, it was similarly measured for the initial and after durability NO x conversions and the SO 2 elimination temperature. These results are set forth in Table 3.

Except that an Al 2 O 3 powder and a TiO 2 powder were mixed so as to be Al 2 O 3 :TiO 2 =2:1 by molar ratio instead of the Al—Ti composite oxide powder, a pelletized catalyst was prepared in the same manner as Example No. 14, and was similarly measured for the initial and after durability NO x conversions and the SO 2 elimination temperature. These results are set forth in Table 3.

Except that the loading amount of Rh was varied to 0.01% by weight in the powder “A”, a pelletized catalyst was prepared in the same manner as Example No. 14, and was similarly measured for the initial and after durability NO x conversions and the SO 2 elimination temperature. These results are set forth in Table 3.

Except that the loading amount of Pt was varied to 0.2 g in the powder “B”, a pelletized catalyst was prepared in the same manner as Example No. 14, and was similarly measured for the initial and after durability NO x conversions and the SO 2 elimination temperature. These results are set forth in Table 3.

Except that Ba was loaded in an amount of 0.2 mole in the powder “B”, and that the mixing amounts of the powder “A” and the powder “B” were the powder “A”/the powder “B”=150/50, a pelletized catalyst was prepared in the same manner as Example No. 14, and was similarly measured for the initial and after durability NO x conversions and the SO 2 elimination temperature. These results are set forth in Table 3.

Comparative Example No. 8

Except that only the powder “A” was used, a pelletized catalyst was prepared in the same manner as Example No. 14, and was similarly measured for the initial and after durability NO x conversions and the SO 2 elimination temperature. These results are set forth in Table 3.

Comparative Example No. 9

Except that only the powder “A”, whose Rh loading amount was 2% by weight, was used, a pelletized catalyst was prepared in the same manner as Example No. 14, and was similarly measured for the initial and after durability NO x conversions and the SO 2 elimination temperature. These results are set forth in Table 3.

Comparative Example No. 10

Except that only the powder “B” was used, a pelletized catalyst was prepared in the same manner as Example No. 14, and was similarly measured for the initial and after durability NO x conversions and the SO 2 elimination temperature. These results are set forth in Table 3.

Comparative Example No. 11

Except that Pt was loaded in an amount of 4 g in the powder “B”, a pelletized catalyst was prepared in the same manner as Example No. 14, and was similarly measured for the initial and after durability NO x conversions and the SO 2 elimination temperature. These results are set forth in Table 3.

Evaluation

By comparing Comparative Example No. 10 and Comparative Example No. 8 with Example No. 14, it is apparent that the NO x conversion after durability was improved greatly by coexisting the powder “B” and the powder “A”. Note that, like Comparative Example No. 11, the initial NO x conversion was improved by increasing the Pt loading amount on the powder “B”, however, the conversion after durability was not improved so much. Further, like Comparative Example No. 9, when the Rh loading amount on the powder “A” was increased, the initial NO x conversion was improved slightly, however, the conversion retention rate was decreased adversely. Furthermore, in Comparative Example No. 10, the SO 2 elimination temperature was extremely high. Therefore, it is apparent that, by coexisting the powder “A” with the powder “B”, SO 2 was eliminated at lower temperatures so that the NO x storage member could be inhibited from the sulfur poisoning.

Namely, it is apparent that, by the coexistence of the Al—Ti composite oxide and the Rh/ZrO 2 the durability was improved remarkably.

Note that, when Example Nos. 14-17 were compared with Example No. 25, there was an optimum range in the Rh loading amount in the Rh/ZrO 2 powder, and that the optimum range lies in from 0.5 to 2% by weight approximately. Further, by comparing Example No. 14 with Example No. 24, it is apparent that the NO x conversion after durability was improved remarkably by making Al 2 O 3 and TiO 2 into a composite oxide. Furthermore, when Example No. 14 and Example No. 26 are compared, 0.2 g of Pt loading amount on the powder “B” was insufficient so that the initial NO x conversion was low, and when Example No. 14 is compared with Example No. 23, it is understood that the honeycomb-shaped monolithic catalyst exhibited a higher performance than the pelletized catalyst.